The OP is referring to a specific mechanic whereby every shield has a significant delay (>0.5s) after releasing a block until they are able to block again (I'm not talking about the shield raise animation, which still occurs, I'm talking about the block actually being active). This mechanic is independent of shield skill, shield speed, etc., and it is probably the #1 thing that kills experienced shielders. It's the reason dueling with a shield is boring, because you can't ever drop your block while your opponent is holding an attack. When I'm fighting other shielders, I rarely even bother to switch to my axe, because it's faster to kill them just by holding sword attacks to exploit this mechanic. Just wait for them to get bored and temporarily drop their block, then release. This tactic isn't as effective against non-shields, because they can drop their block and still recover in time to block anyway.
The neutral nudge is the only thing I've found that can break the monotony, but it's risky due to the wonky nudge hitbox. The chamber technique that Roly mentioned is probably even more risky, and more of a gimmick to mess up top-tier players than a strategy you can rely on.
Since no dev even knows about this mechanic, I'm not going to hold my breath about it being fixed.